The invention relates to a method for monitoring the air pressure of the tires of a motor vehicle utilizing a tire pressure control system. The control system has air pressure control devices on respective ones of the wheels of the motor vehicle and a central unit. The method includes method steps wherein each tire pressure control device measures the air pressure of the tire assigned thereto at time intervals; each tire pressure control device transmits a data transmission to the central unit at time intervals with the data transmission containing at least the measured air pressure and an individual identifier characterizing the tire pressure control device and a transmission pause takes place between two sequential data transmissions of a specific tire pressure control device; and, the central unit compares the transmitted air pressure with an air pressure stored therein and generates a warning signal when the transmitted air pressure deviates beyond the stored air pressure by a pregiven amount.
The invention also relates to a motor vehicle having a tire pressure control system with which the method can be carried out.
For reasons of safety, the tire pressure of motor vehicles must be checked regularly and this is often neglected by the driver of the motor vehicle for various reasons. For this reason, tire pressure control systems have already been developed which include a tire pressure control device on each wheel which automatically measures the tire pressure of the tires of the motor vehicle and announces to the driver of the motor vehicle at least a critical deviation from a desired tire pressure. The tire pressure control devices can, for example, be vulcanized into the tires or can be mounted with adhesive or can be attached on or in the valve or on or in the wheel rim. Appropriate configurations are known.
German patent publication 4,205,911 discloses a method for carrying out the measurement of the air pressure of the tires of a motor vehicle utilizing a tire pressure control system of the kind mentioned above. The tire pressure control system includes tire pressure control devices assigned to corresponding ones of the tires of the motor vehicle and these control devices transmit a high-frequency signal at regular time intervals to a central unit. The high-frequency signal includes at least a measured pressure signal and an individual identifier. The data, which are transmitted to the central unit, cannot be mistaken for data which are, for example, transmitted from another motor vehicle because of the transmission of an individual identifier. In the central unit, value pairs of the form (identifier of the tire pressure control device/wheel position) are stored for each wheel of the motor vehicle so that a conclusion can be drawn in the central unit via a corresponding comparison as to which identifier is transmitted with the corresponding pressure signal from which wheel position of the motor vehicle. A deviation of the transmitted pressure signal from a pregiven value at a wheel position is displayed by the central unit to the driver of the motor vehicle so that the driver can initiate appropriate measures.
The tire pressure control system disclosed in German patent publication 4,205,911 affords the advantage that the tire pressure control devices are mounted directly on the wheels of the motor vehicle and, in this way, the air pressure in the tires can be measured directly and can be transmitted contactlessly by a high-frequency signal to the central unit. A specific frequency of 434 MHz is used at the present time for the high-frequency signal. This frequency is also used by other transmitters such as weather stations which have, in part, a very high transmitting power. If a motor vehicle, which is equipped with a tire pressure control system of the above type, is in the vicinity of such a transmitter, then it can occur that the high-frequency signals, which are transmitted by the tire pressure control devices, are superposed with the high-frequency signals transmitted by the xe2x80x9cdisturbingxe2x80x9d transmitter so that, in the central unit, the data, which are transmitted by the tire pressure control devices, can no longer be received without error or not at all. This causes the situation that the monitoring of the air pressure of the tires of the motor vehicle is no longer reliably possible in the region of the disturbing transmitter.
In addition to the above-described disturbance, additional disturbances can occur in the tire pressure control system. The tire pressure control devices are supplied by batteries whose service life is limited so that a tire pressure control device can become disabled after the service life of a battery has expired and the air pressure of the corresponding tire of the motor vehicle can no longer be monitored. Finally, component defects can occur in the tire pressure control system which lead to the disablement of one or more tire pressure control devices.
In summary, it can be seen that short-term disturbances (caused by a disturbance transmitter) as well as long-term disablements can occur which require maintenance (for example, caused by a run-down battery) in a tire pressure control system such as disclosed in German patent publication 4,205,911. A driver of a motor vehicle no longer takes a warning seriously if a disablement is displayed for each short-term disturbance of the tire pressure control system and the driver later determines that the system functions without error (for example, when the driver has left the region of the disturbing transmitter). If then a long-term disablement occurs in the tire pressure control system (for example, because of a run-down battery), the driver of the motor vehicle could fail to consider a corresponding warning and continue to drive over a long time span with a damaged tire pressure control system. In this case, the safety of the motor vehicle is reduced.
It is an object of the invention to provide a method for monitoring the air pressure of the tires of a motor vehicle utilizing a tire pressure control system wherein erroneous indications of a disturbance of the operation of the system can be avoided to the greatest extent possible. It is also an object of the invention to provide a motor vehicle having a tire pressure control system with which the method can be carried out.
The method of the invention is for monitoring the pressure of the tires of a motor vehicle utilizing a first tire pressure control system which includes a central unit and a plurality of tire pressure control devices on corresponding ones of the wheels of the motor vehicle for respective ones of the tires. The method includes the steps of: utilizing each one of the tire pressure control devices to measure the air pressure in the tire corresponding thereto at time intervals; causing each one of the tire pressure control devices to transmit a data transmission to the central unit at time intervals with the data transmission containing at least the measured air pressure and an individual identifier characterizing the one tire pressure control device and with a transmission pause being present between two sequential data transmissions of a specific one of the tire pressure control devices; monitoring each of the transmission pauses in the central unit which elapses after the receipt of a data transmission which is transmitted by a specific one of the tire pressure control devices; monitoring the air pressure of the tires of the motor vehicle utilizing a second tire pressure control system at least when at least one of the transmission pauses exceeds a pregiven first limit value; comparing the transmitted air pressure to an air pressure stored in the central unit; and, causing the central unit to generate a warning signal when the transmitted air pressure deviates from the stored air pressure beyond a pregiven amount.
A transmission pause, which is present between two sequential data transmissions of a specific tire pressure control device, is defined herein as the time which elapses starting with the receipt of a data transmission of a specific tire pressure control device in the central unit up to the receipt of the next data transmission of the same specific tire pressure control device in the central unit. A transmission pause occurs in that a specific tire pressure control device transmits data transmissions to the central unit in time intervals. A transmission pause also occurs in that a specific tire pressure control device becomes disabled after the transmission of a data transmission to the central unit (for example, because the battery is exhausted) and thereby transmits no further data transmissions. A third possibility of how a transmission pause occurs is that a specific tire pressure control device has sent out data transmissions at time intervals but these data transmissions could not be received by the central unit (for example, because the motor vehicle equipped with the tire pressure control system is located in the region of a disturbance transmitter). The central unit cannot decide for which reason the transmission pause occurs from the above-mentioned possibilities. The central unit only initiates the measures set forth above when a transmission pause exceeds the pregiven first limit value.
The basic idea of the invention is that a second tire pressure control system is provided ancillary to the first tire pressure control system which has tire pressure control devices which transmit the measured air pressure in the form of high-frequency signals to the central unit. The second tire pressure control system has an operation which is independent of the first tire pressure control system and transmits no data via high-frequency signals. In this way, the second tire pressure control system cannot temporarily be disturbed by a disturbance transmitter. The second tire pressure control system is preferably a simply configured cost effective system which reliably monitors the tire pressure of the wheels with lesser accuracy than the first tire pressure control system. The second tire pressure control system can, for example, be a tire pressure control system wherein the air pressure of the tires is determined from the extent of the roll-off or the air pressure of the tires is determined from their vibration behavior. Tire pressure control systems wherein the air pressure from the extent of the roll-off are disclosed, for example, in European patent publications 0,647,536 and 0,675,812 as well as German patent publication 4,400,913. Tire pressure control systems wherein the air pressure of the tires is determined from their vibration behavior are disclosed, for example, in German patent publications 197 23 037 and 4,330,617 as well as U.S. Pat. No. 5,753,809 incorporated herein by reference.
The advantages achieved with the invention are especially that a monitoring of the air pressure of the tires of the motor vehicle is ensured by the second tire pressure control system when there is a disturbance in the first tire pressure control system. For this reason, the safety of the motor vehicle is ensured at all times. In this way, a disturbance of the first tire pressure control system need not be announced to the driver of the motor vehicle and an erroneous indication of a disturbance of operation is precluded.
According to another feature of the invention, a warning signal is generated when at least one transmission pause exceeds a pregiven second limit value which is significantly greater than the first pregiven limit value. The second limit value is preferably so selected that a short-term disturbance of the first tire pressure control system is most likely canceled (for example, because the motor vehicle has left the region of the disturbing transmitter) within the time span which is pregiven by this limit value. A time is pregiven as the second limit value which lies in a range between 10 to 60 minutes. The advantage of this embodiment is that most likely only long-term disturbances of the first tire pressure control system are displayed to the driver of the motor vehicle which make maintenance or repair necessary so that fault displays of a disturbance of operation are substantially precluded. Temporary disturbances in which the air pressure of the tire is monitored with the second tire pressure control system are, in contrast, not displayed to the driver of the motor vehicle so that the driver is not confused by such displays.
According to another feature of the invention, a rotation sensor is permanently assigned to each wheel of the motor vehicle. The second tire pressure control system monitors the air pressure of the tires by means of the rotational sensor signals are known, for example, from European patent publications 0,647,536 and 0,675,812 as well as German patent publication 4,400,913. The advantage of this embodiment is seen in that almost any modern motor vehicle includes rpm sensors which are part of a slip control system. Accordingly, the motor vehicle requires no components, which would not already be available, in order to carry out the method of the invention.
According to still another embodiment of the invention, the air pressure of the tires of the motor vehicle is monitored as follows: permanently assigning rpm sensors to corresponding ones of the wheels of the motor vehicle; establishing at least one characteristic field utilizing the rpm sensor signals and the air pressures transmitted by the tire pressure control devices; when there is a disablement of a specific tire pressure control device, utilizing the characteristic field and the rpm sensor signals and the air pressures transmitted by the remaining tire pressure control devices to at least approximately determine the air pressure of the tire corresponding to the tire pressure control device which has become disabled; and, when the transmission pause of a specific tire pressure control device exceeds the pregiven first limit value, at least approximately determining the air pressure of the corresponding tire with the aid of the characteristic field, the rpm sensor signals and the air pressures transmitted by the remaining tire pressure control devices.
When one or more tire pressure devices become disabled, it is to be understood here and in the following that the central unit receives no data from this tire pressure control device. This can be because the function of the tire pressure control device is disturbed or that the vehicle is in the region of a disturbing transmitter.
The further embodiment utilizes the situation that mostly the two tire pressure control systems operate at the same time and therefore the above-mentioned characteristic field can be prepared with high accuracy. The advantage of the further embodiment is seen in that the accuracy of the second tire pressure control system, which determines the air pressure of the tires from the rpm sensor signals, can be greatly increased.
According to another feature of the invention, the method includes the further steps of: permanently allocating an rpm sensor to each one of the wheels; forming at least one characteristic field from the signals transmitted by the rpm sensors and the air pressure transmitted by the tire pressure control devices; determining whether the air pressure in the tires is adequate or whether the air pressure in at least one of the tires is not adequate by utilizing the characteristic field when knowing the rpm signals when several of the tire pressure control devices are disabled; and, when the transmission pauses of several of the tire pressure control devices exceed the pregiven first limit value, determining, with the aid of the characteristic field, whether the air pressure in the tires of the motor vehicle is adequate or whether the air pressure in at least one tire is not adequate.
The formulation that the air pressure in the tires of the motor vehicle is sufficient is understood to mean that the air pressure of each tire of the motor vehicle does not deviate from a pregiven desired air pressure by more than a pregiven value. This embodiment too makes use of the condition that mostly both tire pressure control systems operate simultaneously and the above-mentioned characteristic field can therefore be prepared with high accuracy. The advantage of this further embodiment is seen in that for the case wherein the transmission pause of several tire pressure control devices exceeds the pregiven first value, the reliability of the statement as to whether the air pressure in the tires of the motor vehicle is sufficient or whether the air pressure in at least one of the tires is insufficient can be considerably increased.